|Publication number||US7141943 B2|
|Application number||US 11/026,148|
|Publication date||Nov 28, 2006|
|Filing date||Dec 30, 2004|
|Priority date||Dec 30, 2004|
|Also published as||US20060145639|
|Publication number||026148, 11026148, US 7141943 B2, US 7141943B2, US-B2-7141943, US7141943 B2, US7141943B2|
|Inventors||Joong-Ho Song, Ick Choy|
|Original Assignee||Korean Institute Of Science And Technology|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (14), Referenced by (25), Classifications (10), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention generally relates to a Brushless DC (BLDC) motor system, and more particularly to a BLDC motor system and a method of controlling the same to reduce a commutation torque ripple, which occurs during a commutation interval.
Generally, a brushless DC (BLDC) motor is a type of a DC motor that does not use mechanical contacting units such as a brush and a commutator. The BLDC motor is known to use an electrical rectifier in lieu of the mechanical contacting units. A typical BLDC motor consists of two main portions, namely, a stator and a rotator. The stator is formed of a plurality of coils, which receive poly-phase currents (e.g., three-phase currents). Rotating magnetic fields are formed by the currents applied to the coils. The rotator is formed of at least one permanent magnet surrounded by the stator, wherein the rotator rotates within the stator.
To rotate the BLDC motor continuously, the rotating magnetic fields must be formed continuously in the BLDC motor. More specifically, in order to rotate the rotator, a commutation of currents applied to the coils must be performed at a proper timing. Commutation means that the currents applied to the coils are switched. In other words, the direction of the currents becomes changed. For example, three-phase currents are commutated in sequence in accordance with the angular positioning of the rotator. Therefore, the angular positioning of the rotator must be measured in order to perform a proper commutation. For this purpose, a position sensor, such as a hall sensor or a resolver, is required.
The BLDC motor has certain advantages since there is no need to replace a worn brush with a new one, the noise caused by an Electromagnetic Interference (EMI) is reduced, the heat transfer characteristic is fair, and a higher power can be obtained compared to other motors having the same size.
Generally, a rectifier circuit, which simply converts AC power into DC power, may be used as the converter 20 for driving the BLDC motor 50.
The inverter 30, which includes six switching elements, is a power transducer for converting DC power into three-phase AC power. There are two types of inverters, i.e., a current source inverter and a voltage source inverter. The voltage source inverter is mainly used for driving the BLDC motor 50. That is, the voltage source inverter receives DC power and applies voltages to the BLDC motor 50 in a form of a pulse string.
The controller 80 controls the application of the three-phase currents from the inverter 30 to the stator 51 for rotating the rotator 52. This is accomplished by synchronizing the magnetic poles of the rotator 52 with the magnetic poles of the stator such that the operation of the BLDC motor 50 is controlled. More particularly, the controller 80 controls the operation of the BLDC motor 50 by applying a voltage and a current in the form of pulses, the widths of which are modulated with saw-tooth wave signals inputted from the outside. The controller 80 detects the current applied to the BLDC motor 50, generates a control signal by calculating the signals inputted from the position sensor 60 and the speed sensor 70 based on the value of the detected current, and supplies the control signal to the inverter 30. Such control by the BLDC motor is often called the torque control since the BLDC motor 50 controls the torque, which occurs by the current supplied to the BLDC motor 50.
According to the conventional method adopting the switching operation of the inverter 30, the currents inputted to the BLDC motor are controlled through the use of the three-phase currents outputted from the inverter 30 for reducing a torque ripple. That is, the switching elements of the inverter 30 are used to apply three-phase voltages to the BLDC motor 50. In such a case, the BLDC motor 50 is controlled by a constant duty ratio of pulses for switching. In other words, the current of each phase is applied to the coil whenever the rotator is detected to be at a predetermined position, for example, at an angle of 120° from a reference point. Also, it takes time to increase the current to a desired value or to decrease the current to zero. That is, the increase or decrease of the current is delayed due to a resistor and an inductance of each coil. The current delay, as well as the changes of a rotational angle, generates torque. The torque ripple is generated by the commutation whenever the phase of the current applied to the coil is changed.
The torque ripple causes vibrations and noises in the motor. To reduce the torque ripple in the BLDC motor 50 using a trapezoid wave of counter-electromotive force, it is preferable to use three AC current sensors 40 for controlling the currents outputted from the inverter 30. There is currently a method of reducing the torque ripple using a single DC current sensor for controlling the output currents of the inverter instead of using three current sensors.
According to the conventional methods for reducing the torque ripple using a single DC current sensor, the switching elements of the inverter 30 are switched with a constant duty ratio of pulses during the commutation interval, or the turn-off time of the switching elements of the inverter 30 is delayed. However, the conventional methods are disadvantageous since they are undesirably sensitive to the change of parameters of the BLDC motor. Thus, the stable performance of the BLDC cannot be guaranteed at the low or high speed due to the speed change of the BLDC motor 50.
Therefore, it is an object of the present invention to address and overcome the above-referenced problems of the prior art. In particular, the object of the present invention is to reduce a torque ripple, which occurs during a commutation interval, by changing a duty ratio of pulses according to a speed change of the BLDC motor.
Further, another object of the present invention is to control the operation of the BLDC motor without an additional sensor or apparatus.
According to one aspect of the present invention, the present invention provides a Brushless DC (BLDC) motor system, comprising: a power supplying means for supplying a DC voltage and a DC current; a BLDC motor including a rotator and a stator having a plurality of coils, wherein a position change of the rotator generates a counter-electromotive force, and currents having different phases are applied to the coils; an inverter for receiving the DC current and providing the currents having the different phases to the coils, wherein the inverter includes a plurality of switching means driven with different switching pulses during a commutation interval to rotate the rotator; a DC current sensing means for sensing the currents when the currents are applied from the switching means to the coils; a position detecting means for detecting a position of the rotator; and a controlling means for receiving a reference current from outside and calculating a magnitude of the counter-electromotive force based on the detected position, generating a compensation voltage based on the counter-electromotive force, and controlling the switching means based on the compensation voltage during the commutation interval, wherein the controlling means controls a duty ratio of the switching pulses based on differences between the currents applied to the coils and the reference current.
According to another aspect of the present invention, there is provided a method of controlling a Brushless DC (BLDC) motor, which includes a rotator, a stator having a plurality of coils, and a plurality of switching means for switching currents from a power supplying unit to the coils. The switching means is driven with different switching pulses. Such method comprises the steps of: detecting a position of the rotator in the BLDC motor, wherein a position change of the rotator generates a counter-electromotive force; detecting the currents when the currents are applied from the switching means to the coils; calculating a magnitude of the counter-electromotive force based on the detected position; generating a compensation voltage based on the magnitude of counter-electromotive force; and controlling the switching means based on the compensation during the commutation interval, wherein the controlling step comprises the step of controlling a duty ratio of the switching pulses applied to the switching means based on differences between the currents applied to the coils and a reference current value.
The above object and features of the present invention will become more apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings.
With reference to the drawings, the preferred embodiments of the present invention will be described below in detail.
The BLDC motor system controls the BLDC motor 300, which includes a stator 310 and a rotator 320. The stator 310 has three coils and the rotator 320 has at least one permanent magnet surrounded by the stator 310.
The inverter 200 receives DC power from a power supply 100. It is preferable that the power supply 100 further comprises a converter (not shown) for converting AC power, which is inputted from the outside, into DC power. The inverter 200 comprises six switches S1 to S6. The switches S1 to S6 can be formed with various switching elements. It is preferable to form each switch with a bipolar transistor. The inverter 200 receives DC power necessary for operation from the power supply 100, while the switches S1 to S6 are switched by switching the signals from the DC current controller 500. The inverter 200 further comprises a single DC current sensor 210 for detecting currents (ia, ib and ic) applied to the coils of the stator 310. The switching order of the switches in the inverter 200 is equal to that of the conventional 3-phase inverters. As such, the switching order will not be explained in detail.
The rotator 320 in the BLDC motor 300 rotates as the currents having three-phases (e.g., a-phase, b-phase and c-phase) are applied to the coils of the stator by the switching operation of the inverter 200.
The position sensor 400 detects a change of the magnetic field due to the rotation of the rotator 320. In other words, the position sensor 400 detects a position of the rotator, generates a position sensing signal θ of the rotator 320, and then transmits the position sensing signal (e.g., phase angle θ) to the DC current controller 500. In the preferred embodiment of the present invention, the position sensor 400 may be formed with a hall sensor.
The DC current controller 500 receives the position sensing signal from the position sensor 400, calculates a counter-electromotive force E generated by a change of phase angle θ of the rotator 320, receives the reference current idref from the outside and the detection current |id|from the DC current sensor 210 via a rectifier 600, generates a Pulse Width Modulation (PWM) carrier wave for driving each switch S1 to S6 of the inverter 200, and transmits the PWM carrier wave to the inverter 200. When saturation voltage Vsat is generated during a commutation interval, the DC current controller 500 compensates the saturation voltage Vsat appropriately and generates switching signals for reducing the torque ripple, which occurs during the commutation.
The first adder 501 receives the reference current idref from the outside, preferably a microcomputer (not shown), and the detection current |id| from the rectifier 600, which rectifies the current detected by the single DC current sensor 210. The first adder 501 calculates an error between the reference current idref and the detection current |id|. The first multiplier 502 multiplies a current inputted from the first adder 501 by a first constant K. The second multiplier 503 receives the counter-electromotive force E, which is generated by based on the phase angle change of the rotator 320 from the position sensor 400, and multiplies the counter-electromagnetic force E by a second constant (preferably 2). The second adder 504 receives and adds outputs of the first multiplier 502 and the second multiplier 503. The voltage compensator 505 generates the compensation voltage Vcomp for reducing the torque ripple on the basis of counter-electromotive force E, which is generated by the phase angle changes of the rotator 320 in the BLDC motor 300.
Referring now back to
Referring now to
For example, the operations of the BLDC motor system will be described in case that the states of the switches S1 and S3 in the inverter 200 are transited from the on state into the off state and from the off state into the on state, and the state of switch S2 is maintained at the on-state during the commutation interval. At this time, the phase of the current in the inverter 200 is changed from a-phase to c-phase, and then is changed from b-phase to c-phase by the commutation. At the commutation interval, the current ia of a-phase is decreased and the current ib of b-phase is increased, while the current ic of c-phase has the previous current magnitude.
By coinciding zero-point reach time tc of current ia with final-point reach time tf of current ib in order to reduce the torque ripple, the commutation of the phase current of BLDC motor 300 can be maintained uniformly, provided that the DC current controller 500 controls the switch S3 and switch S2 with the duty ratio of pulses determined by equation 1:
wherein, E is the counter-electromotive force and Vd is a DC supplying voltage. That is, the final-point reach time tf of current ib coincides with the zero-point reach time tc of current ia by switching the switches S3 and S2 at the duty ratio of pulses determined by equation 1. This is because the final-point reach time tf of current ib is shorter than the zero-point reach time tc of current ia during the commutation interval. According to equation 1, the pulse width denoting the turn-on time of switches S2 and S3 becomes the duty ratio x the sampling time Dlow×Ts, as shown in
wherein E is the counter-electromotive force and Vd is a DC supplying voltage. The state of switch S2 is converted from the off state into the on state, and the state of switch S3 is maintained at the conduction state. The DC current controller 500 makes the zeropoint reach time tc of current ia coincide with the final-point reach time tf of current ib by controlling switch S1 of the inverter 200 with the duty ratio determined by equation 2. This is because the zero-point reach time tc of current ia is shorter than the final-point reach time tf of current ib. If the duty ratio at the low and high speed regions is calculated by equations 1 and 2, the compensation voltage V, for reducing the torque ripple during the commutation can be calculated by the following equations 3 and 4 based on the calculated duty ratio. The relationship between the compensation voltage and the switching duty ratio in the low-speed region is represented by equations 3 and 4:
V comp=2V d(1−D low) Eq. (3)
V comp=2/V d D high Eq. (4)
On the contrary, if variable 4E/Vd according to the speed of the BLDC motor 300 is increased over 1, then the compensation voltage Vcomp is increased by equation 4. This is because the switching duty ratio Dhigh in the high-speed region is increased by equation 2. That is, if the speed of the BLDC motor 300 is increased in the high-speed region, then the magnitude of counter-electromotive force E is increased since the angular velocity of the rotator of the BLDC motor 300 becomes large. In such a case, the voltage compensator 505 increases the compensation voltage Vcomp calculated by equation 4 since the switching duty ratio is increased by equation 2.
According to the present invention, the torque ripple generated during the commutation interval can be reduced with a single DC current sensor.
The present invention is described with reference to the preferred embodiments and the drawings, but the description is not intended to limit the present invention to the form disclosed herein. It should also be understood that a person of ordinary skill in the art is capable of using a variety of modifications and other embodiments equivalent to the present invention. Therefore, only the appended claims are intended to limit the present invention.
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|U.S. Classification||318/400.34, 318/400.04, 318/599|
|International Classification||H02K21/00, H02P1/18, H02P3/08, H02K23/00, H02K31/00|
|Dec 30, 2004||AS||Assignment|
Owner name: KOREAN INSTITUTE OF SCIENCE AND TECHNOLOGY, KOREA,
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SONG, JOONG-HO;CHOY, ICK;REEL/FRAME:016150/0318
Effective date: 20040930
|Jul 5, 2010||REMI||Maintenance fee reminder mailed|
|Nov 28, 2010||LAPS||Lapse for failure to pay maintenance fees|
|Jan 18, 2011||FP||Expired due to failure to pay maintenance fee|
Effective date: 20101128